1. Field of the Invention
This invention relates to a length measuring device capable of measurement with high accuracy and with reduced temperature deviations.
2. Description of the Related Art
As one of length measuring devices for precision measurement, the dial gauge is used widely in the machine industry. The main body of a dial gauge is secured at a fixed position with respect to a reference plane and the probe provided extending outward from the main body is applied to an object under measurement. The distance moved by the probe or the amount of retract of the probe is transmitted to the indicator of the main body to make the indicator needle to deflect. This type of length measuring device can make measurements of the order of 1/1000 mm. Dial gauges are used also for measurement of the diameter and roundness of the object under measurement.
Among the dial gauges, there are digitized types using a Colpitts oscillator that have appeared recently. To be more specific, the three-piece coil of a Colpitts oscillator is disposed along the moving area of a core attached the probe. In this case, the coil in the middle is for common use and is used to form a series circuit with either of the coils on both sides. In measurement, these series circuits are used as the reactance of a Colpitts oscillator and oscillation frequencies are obtained for two cases: one is that the center coil forms a series circuit with one of the coils on both sides and the other case is that the center coil forms a series circuit with the other coil.
The oscillation frequencies obtained with a Colpitts oscillator as described above are counted by a counter. The numbers counted are supplied to a microcomputer. The microcomputer calculates the displaced amount of the core, that is, the distance moved by the probe using the difference of the two counted numbers supplied, namely, the difference in frequency corresponding to a variation in reactance according to the position of the core. And the distance moved thus calculated is sent to a display unit for digital display.
This type of length measuring device operates on the principle that the variation in frequency is found by utilizing the variation in inductance according to the position of the core. Therefore, the reliability of measurement is influenced by the temperature characteristics of the coils, which can be said to be the heart of the length measuring device. The temperature characteristics can be investigated in advance. Hence, if the current temperature is known, the influence can be compensated. For this purpose, it is necessary to measure the temperature. For portable, compact length measuring device, a thermistor is generally adopted for temperature measurement after various conditions including convenience of mounting are taken into consideration.
Thermistors are expensive, which is a problem in adoption. In addition, for measuring temperature with a thermistor, special measuring circuit is required. Thus, the use of does not make for the reductions in size and cost of the length measuring device.
A well-known method of compensating the temperature characteristics of the coils is use of capacitors which have approximate but opposite temperature characteristics. Such capacitors are special types and consequently, expensive and hard to come by.